The present invention relates to an innovative method and a plant applying this method for the production of sheets of paper material having a substantially stiff structure, e.g. suitable for being used in replacement of the conventional undulated cardboard for the production of boxes, packing and the like.
The present invention also relates to the paper product produced by this method.
In the prior art the so-called undulated cardboard formed from one or more internal layers of undulated paper of different thickness and from flat sheets glued on the two faces of each undulated layer is well known. This structure has the quality of supplying stiffness with minimal use of material. On the other hand production is relatively complicated as it is necessary to produce a normally relatively thick paper by conventional paper production methods. This paper must then be undulated, covered with glue and covered on both faces with flat sheets of other paper which is also produced by conventional methods. Lastly, everything has to be moderately pressed to ensure gluing, cut into panels and allowed to dry. There is accordingly relatively high production complexity which must be undergone to undulate the paper, position it, glue the flat sheets together et cetera. It is always necessary to produce the starting paper with the known problems originating in the conventional paper production.
A feature of undulated cardboard which is sometimes a disadvantage is the different stiffness which it offers in the direction of extension of the undulations with respect to the orthogonal direction. To obviate this, cardboard made up of several undulated layers glued together with the undulations arranged in different directions has been proposed. This further complicates undulated cardboard production. In addition it must be remembered that continuous production is possible only when the undulation is executed with the folds extending transversely to the production line running direction or at best with minimal inclination with respect to said running direction.
The U.S. Pat. No. 5,290,621 discloses the production of individual corrugated wafer board panels made in three steps from wooden fragments combined with thermosetting resin. Of course, this basic material is different from the paper material used in the technical field of the present invention and, according thereto, its behaviour in the manufacturing steps and the features of the final product are quite different.
The general purpose of the present invention is to remedy the above mentioned shortcomings by making available a paper material in sheets having stiffness comparable to that of undulated cardboard, minimal material expenditure, reduced cost and production ease. Another purpose of the present invention is to make available a production method and plant applying said method.
In view of this purpose it was sought to provide in accordance with the present invention a method for the continuous production of sheets with substantially stiff structure of paper material comprising the steps of paper grinding to obtain paper flakes, spreading the flakes to obtain a carpet of flakes, sprinkling the carpet with gluing liquid, accomplishing a first carpet compression to produce a first compacting of the flakes while holding a virtually flat carpet configuration, accomplishing a second carpet compression to produce another compacting of the flakes, and performing during this second compression a simultaneous forming to obtain a continuous production of a sheet with a plurality of stiffening dishes having an elongated form and arranged in at least two different directions on the sheet expanse.
Again in accordance with the principles of the present invention it was sought to provide a plant for the continuous production of sheets with substantially stiff structure of paper material comprising paper grinding means for obtaining paper flakes, distribution means for spreading the flakes in a uniform layer to form a carpet of flakes, sprinkling means for sprinkling the carpet with gluing liquid, uniform pressing means for subjecting the carpet to a first compression to produce a first compacting of the flakes while holding an essentially flat carpet configuration, and shaped pressing means for subjecting the carpet to a second compression to produce a second compacting of the flakes with the shaped pressing means executing during said second compression a simultaneous shaping to obtain a continuous production of a sheet with a plurality of stiffening dishes having an elongated form and arranged in at least two different directions on the sheet expanse.
Lastly, again in accordance with the principles of the present invention it was sought to provide a sheet with substantially stiff structure of paper material composed of pressed paper flakes, characterized by comprising a plurality of stiffening dishes having an elongated form and arranged in at least two different directions on the sheet expanse.